3. Catalytic hydrogenation
The addition of hydrogen to an unsaturated compound in the
presence of catalyst to obtain saturated compound is called catalytic
hydrogenation.
The hydrogenation is a addition reaction.
The Hydrogenation is reduce double and triple bond in hydrocarbons.
The hydrogenation of alkenes is an exothermic reaction.
4. What is the need
of catalyst?
The hydrogenation of
alkenes and alkynes with
molecular hydrogen does
not occur at room
temperature without a
catalyst but often take
place at room temperature
if metal catalyst is added.
Also the catalyst is able to
provide a new pathway for
the reaction with a lower
free energy of activation.
5. Catalyst used in catalytic hydrogenation
reaction are of following:-
1. Palladium (Pd)
2. Adam's catalyst (platinum dioxide, PtO2)
3. Raney nickel
4. Copper chromite (Cu2Cr2O5)
5. Rhodium(Rh)
6. Ruthenium (Ru)
7. Trimethylamine (N(CH3)3)
6. 1. Heterogeneous catalytic hydrogenation
The catalysts is not soluble in the reaction media, thus this process is
referred to as a heterogeneous catalysis.
The catalyst assists in breaking the pi bond of the alkene and the H-H
bond.
2. Homogeneous catalytic hydrogenation
The catalyst is soluble in the reaction media, thus this process is
referred to as a homogeneous catalyst.
7.
8. Mechanism of catalytic hydrogenation
Step 1:
hydrogen molecules react with the metal atoms at the catalyst surface.
9. Step 2:
The relatively strong H-H sigma bond is broken and replaced with two
weak metal-H Bond.
10. Step 3:
the pi bond of the alkene interacts with the metal catalyst weakening
the bond.
11. Step 4:
A hydrogen atom is transferred from the catalyst surface to one of the
carbons of the double bond
12. Step 5:
A second hydrogen atom is transferred from the catalyst surface forming
the alkane.
The alkane is released from the catalyst’s surface allowing the catalyst to
accept additional hydrogen and alkene molecules.
13. Factor affect the hydrogenation reaction are;
1. Catalyst surface
2. Temperature
3. Pressure
1. Catalyst surface
The speed of a hydrogenation will depend on the type and amount
on active surface available.
Increasing the ratio of catalyst to the substance undergoing
hydrogenation usually increase the speed of the hydrogenation.
The reaction takes places on the surface of the catalyst thus the rate
of the reaction is proportional to the surface area of the catalyst
14. 2. Pressure
The rate of reaction is generally increased by increasing pressure
because a gas phase is usually present and increased pressure gives
increased concentration.
This is the simple application of the mass-action law, or le chatelier
principle.
3. Temperature
Increasing temperature has adverse affect on catalyst.
So the catalyst activity decreases and resultant rate of reaction
decreases.
15. Advantages and disadvantages
Relatively low reaction temperatures
Relatively high specificity
Far more easily studied from chemical and mechanistic aspects
Far more active
Generally far more selective for single product
Disadvantages
More difficult for achieving product/catalyst separations.
16. Applications
Hydrogenation is used in many industrial applications.
Ex: hydrogenation is used to convert alkenes into alkanes(paraffins) and
cycloalkanes.
It is also used to prepare vegetable ghee from vegetable oil.
Margarine is a semi-solid butter substitute created from vegetable
oil.
